A passive optical network (PON) is used in an access network for delivering broadband services such as Internet access, digital television and telephone service, to residential and business subscribers (i.e., customers of the service provider). The essence of a PON is that nothing but optical fiber and passive components are found in the path between the central office and subscribers. A single fiber can run from the central office to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers. The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). The IEEE has also adopted Ethernet-based (i.e., IEEE 802.3-based) PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). These standards and recommendations are known to persons skilled in the art to which the invention relates and are therefore not described in further detail in this patent specification. Although the term GPON may be used herein for convenience with regard to embodiments of the present invention described below, the invention can be applied to any suitable PON technology.
As illustrated in FIG. 1, in a related art arrangement for delivering services to subscribers, a PON 10 includes an Optical Line Terminal (OLT) 12 (also known as optical line terminator) optically coupled to a number of Optical Network Terminators (ONTs) 14 (also known as optical network terminals and optical network units) by an arrangement of optical fibers 16 that includes one or more optical splitters (not separately shown for purposes of clarity). Each ONT 14 can be connected to various customer premises equipment (CPE) 18, such as a residential local area network (LAN) router, telephone, television, etc.
In the downstream direction, i.e., data transmitted from OLT 12 (e.g., located at the central office) to an ONT 14 (e.g., located at a subscriber's premises), the data packets are broadcast from OLT 12 to all of ONTs 14, and an ONT 14 can select the data to receive by matching the address embedded in the data packets to a previously provisioned or learned address. In other words, an ONT 14 only “listens” to data packets having a matching address. Thus, OLT 12 can transmit data “downstream” to a particular or selected ONT 14 by addressing it to that ONT. In the “upstream” direction, i.e., data transmitted from an ONT 14 to OLT 12, the data packets are time-division multiplexed.
In a GPON system, the realizable upstream data Reed Solomon Forward Error Correction (RS FEC) optical coding gain attainable is related to the ability to recover the delimiter field correctly. The delimiter field is an embedded piece of information placed there to assist an upstream OLT data receiver in aligning the recovered framing. If the delimiter field is not found or incorrectly found, the entire upstream data burst arriving at the OLT from the ONT is recovered incorrectly and subsequently lost. This situation results in multiple transmissions required to move upstream data from the ONT to the OLT.
Existing OLT solutions utilize a delimiter search method which looks within multiple recovered framing bytes to find the upstream burst alignment. The existing search method will tolerate at most one bit error within a 2.5 byte delimiter field. The existing search method is repeated independently for every received upstream burst.
Related art existing solutions search into the bit interleaved parity (BIP) block of the upstream framing when looking for the delimiter. This BIP value changes every upstream burst and is dependent on the actual data transmitted in the previous upstream burst. This opens up the possibility that the changing BIP values may take on some of the same values as the correct delimiter values but offset in time from the correct delimiter. This is essentially a “false match” if the correct delimiter alignment is experiencing a bit error at this instant or a “multiple delimiter match” condition occurs within the delimiter search range. Either of these results may cause the recovered upstream data to be incorrect.
If the delimiter can not be correctly found within the upstream burst, RS FEC cannot provide increased optical coding gain. Since the GPON standard does not make the delimiter more robust or protect it from error conditions, an optical coding gain which RS FEC may provide cannot be realized. When working correctly, RS FEC allows the optical link bit error rate (BER) to be of lower quality as data bit errors can be corrected for the user data. However, this means that the delimiter which is not covered by RS FEC capability must be recovered under the more stressful operating conditions where bit errors are present.
Related art delimiter recovery approaches do not use knowledge of previous upstream bursts recovered to aid in delimiter recovery. In fact, when multiple delimiter matches are found within the search range, the entire upstream data burst is often discarded.
Current GPON implementations can only recover 0.5 dB or less of RS FEC optical coding gain.